An IDE controller is embodied in one or more Integrated Circuit (IC) chips that are integrated into an IDE peripheral device. The IDE controller handles address, data and control signals by means of which it can receive device control commands from a computer equipped with an IDE interface adapter that is compatible with the Industry Standard Architecture (ISA) computer Input/Output (I/O) bus or the Extended Industry Standard Architecture (EISA) computer I/O bus. The IDE computer interface circuitry is integrated into the motherboard of some computers and circuit boards containing the IDE computer interface circuits are available for installation in a connector slot of an ISA or EISA computer I/O bus. In the present invention the IDE referred to repeatedly is the IDE developed by Western Digital Corporation for Compaq Computer Co., and is fully compatible with ISA or EISA specifications. IDE has been used in the art to denote other and similar systems.
As a subset of its computer interface logic, the IDE controller emulates a prior art disk drive controller standard widely known as ST506, but also known as ST506/412 and ST412. ST506 is the only disk drive controller specification known to the applicant to be supported in the Basic Input Output System (BIOS) Read Only Memory (ROM) software of commonly available BIOS ROMs for AT-compatible computers. This compatibility with disk control software that is built into every AT-compatible computer, along with fewer, or smaller and less complicated, circuit boards and fewer interconnect cables, gives IDE a distinct cost advantage over other disk drive interface technologies. At the time of this application, AT-compatible computers with built-in IDE adapters and magnetic disk drives with built-in IDE controllers are widely available from numerous manufacturers.
A big disadvantage of prior art IDE interfaces is that they are only designed to support the device control and data transfer capabilities of random access magnetic disk drives. Such drives feature a three-dimensional storage geometry of multiple recording surfaces and multiple concentric data tracks, also known in the art as cylinders, using incremental step movable head assemblies to access the tracks. Each track is arbitrarily divided into a number of sectors, each containing a fixed number of data bits.
Another disadvantage of prior art IDE interfaces is that only two drives can be connected to an IDE computer interface. One known as the primary drive, which is generally the "boot" drive, and the other the secondary drive.
Manufacturers of devices that store data using some other geometries, such as the Compact Disk Read Only Memory (CD-ROM) with a single track spiral geometry, have been deterred from taking advantage of the high performance and low cost of the IDE interface, at least in part, by the two-drive limitation of IDE. They have usually developed a proprietary ISA bus interface adapter or have implemented a device controller compatible with the Small Computer System Interface (SCSI) controllers. Some designs incorporate a SCSI controller in the CD-ROM drive chassis and interface the SCSI controller to the computer through an ISA bus parallel printer adapter. Such designs impose an additional electrical and thermal load on the computer power supply and cooling systems and require availability of at least one ISA bus connector slot, thus reducing the number of other ISA bus devices that may be installed in the computer, except in the case of a SCSI compatible device when a SCSI ISA bus adapter is otherwise required in the system. Some less expensive magnetic tape drives are designed to interface with the computer via the floppy disk controller. The low data transfer rate characteristic of floppy disk controllers imposes a severe performance penalty and, as the most common floppy disk controllers are limited to two drives, imposes a configuration problem for many systems.
What is clearly needed is an Enhanced IDE (EIDE) interface that can provide I/O access to addressable block storage locations on devices other than magnetic disk drives such as CD-ROMs, writable Compact Disk devices, and can handle a multiplicity of secondary peripheral devices.